Recently, a white LED has been actively developed and spread into diverse fields, such as illumination, vehicle mounted lamps, liquid crystal backlights, and the like. This white LED, for example, has the characteristic that its on/off-switching response speed is very high in comparison to that of a white light source such as a fluorescent light or the like. Accordingly, a visible light communication system in which the illumination light of a white LED has a data transmission function using white LED light as a data transmission medium has been proposed (see Patent Document 1 below). That is, the luminescence intensity of the white LED is modulated in accordance with the transmission data, and the receiving side converts the intensity of the light into an electric signal through an optical-electrical converter (0/E converter), such as a photodiode (hereinafter referred to as a “PD”) or the like, to realize data transmission.
The white LED, as described in Non Patent Document 1 below, for example, may be mainly classified into three kinds in accordance with the luminescence scheme.
(1) Blue-light-excitation-type white LED: this is obtained by combining a blue LED with a phosphor that emits mainly yellow light. That is, a phosphor that is represented by a YAG (Yttrium Aluminum Garnet) group is arranged around a blue LED, and this arrangement is accommodated in a single package. In this structure, the surrounding phosphor is excited by the blue light output from the blue LED arranged in the center, and the light (mainly yellow) that is mainly complementary to blue is output from the phosphor. A pseudo-white light is obtained by mixing the yellow fluorescence output from the phosphor and the blue light output from the blue LED.
The blue-light-excitation-type white LED has the following advantages: (1) in comparison to other types, it has high energy use efficiency, and high illumination is easily obtained, and (2) due to its simple construction, it can be inexpensively manufactured. On the other hand, the disadvantage of this white LED is that it has bad color rendering. Color rendering indicates the characteristics of color appearance of an object under illumination, and the closer its color is to that perceived under natural light, the better the color rendering.
(2) Ultraviolet-light-excitation-type white LED: This is obtained by combining an ultraviolet light with phosphors that emit three primary colors of R (Red), G (Green), and B (Blue), respectively. That is, phosphors that emit three primary colors of R, G, and B are arranged around an ultraviolet LED, respectively, and this arrangement is accommodated in a single package. In this structure, the surrounding phosphors are excited by the ultraviolet light output from the ultraviolet LED arranged in the center, and the light of three primary colors of R, G, and B is output from the phosphors, respectively. A white light is obtained by mixing the R, G, and B lights.
The ultraviolet-light-excitation-type white LED has the advantage of good color rendering. On the other hand, the disadvantages of this white LED are that (1) it has low energy use efficiency in comparison to the blue-light-excitation-type white LED and it is difficult to obtain high intensity of illumination, and (2) since it emits ultraviolet light, the driving voltage of the LED becomes high.
(3) Three-color-emitting-type white LED: this is obtained by combining LEDs of R, G, and B. That is, three kinds of LED, i.e. a red LED, a green LED, and a blue LED, are accommodated in a single package. In this structure, the white light is obtained by simultaneously making the LEDs emit the three primary colors, respectively.
The three-color-emitting-type white LED has the advantage that it has the advantages of good color rendering like the ultraviolet-light-excitation-type white LED. On the other hand, the disadvantage of the white LED is that since three kinds of LEDs are packaged in a single package, its manufacturing costs become high in comparison to other types of white LEDs.
An optical communication device using the white LED in the related art is as illustrated in FIG. 13(A). If transmission data is provided to a driving unit 902 of a transmitter 900, the corresponding driving current is output to a white LED 904, and the white LED 904 emits light in accordance with the transmission data. For example, the white LED 904 blinks on and off through modulation in a modulation method such as OOK (On-Off Keying) or the like. The light signal output from the white LED 904 is incident to a PD 912 of a receiver 910 to be converted into an electric signal, and then is converted into a voltage signal through a trans-impedance amplifier (current-voltage conversion amplifier) 914. This voltage signal is subjected to a desired equalization processing in an equalizer 916, and then is binarized by a limiting amplifier 918 to be output as reception data.
However, in the case of using the blue-light-excitation-type white LEDas the white LED 904, the response speed of the light output from the phosphor is low, and thus only a transmission speed of about several Mbps at most can be obtained (see Non Patent Document 2 below). Accordingly, a method for aiming at high-speed data transmission by installing an LED color filter which filters only the color blue in front of an optical-electrical converter and removing an optical component having a low response speed, which is output from the phosphor, through this color filter has been proposed (see Patent Document 1). FIG. 13(B) illustrates the configuration of the device in this case, in which a blue color filter 922 is arranged on the light incident side of the PD 912 of the receiver 920. Using this blue color filter 922, the light emitted from the phosphor having a low response speed in the optical signal is removed. Accordingly, only the light of the blue LED is incident to the PD 912, and as a result, data transmission faster than that according to the above-described configuration can be performed. However, even using this method, only a transmission speed of about several 10 Mbps at most can be obtained.
Also, in the case of using the ultraviolet-light-excitation-type white LED as described above as the white LED 904, a transmission speed becomes about several Mbps for the same reasons as the case where the blue-light-excitation-type white LED is used. In addition, since the driving voltage of the LED is increased, it becomes difficult to configure the driving circuit. In this case, there is a method for aiming at improvement in the response speed of the light emitted from the phosphor through the improvement of the phosphor material. However, this method has problems in that it is difficult to obtain the desired intensity of illumination and the cost of the phosphor material itself is increased.
Also, in the case of using the three-color-emitting-type white LED as described above as the white LED 904, there is no phosphor luminescence component in comparison to the above-described methods, and it becomes possible to transmit data by performing wavelength multiplexing whereby the respective LEDs carry different signals to make high-speed transmission possible (see Patent Document 2 below). However, since a plurality of LEDs is used, the manufacturing costs increase.
As seen above, it is very preferable if a general-purpose and cost-advantageous blue-light-excitation-type white LED is used and high-speed transmission can be performed. As an improvement from this viewpoint, there is an “optical communication transmitter or the like” as disclosed in Patent Document 3 below. As briefly illustrated in FIG. 13C, the optical communication transmitter has the configuration in which a peaking circuit 932 is installed in a transmitter 930. Accordingly, the generation and adjustment of an optimum driving current waveform that is most suitable to perform a high-speed modulation is performed, and thereby even in a location where sunlight or light from a fluorescent light exists, an optical signal that is suitable to high-speed transmission is output from the transmitter.